1. Field of the Invention
This invention relates to a viscosimeter for measuring the relative intrinsic or inherent viscosity of a solution in a solvent.
2. Description of the Related Art
According to the state of the art, the difference is made between the relative, the specific as well as the inherent viscosity and finally the limiting viscosity number (intrinsic viscosity). By relative viscosity, we understand the quotient of the viscosity of the solution, for example of a polymer, to the viscosity of the pure solvent. The inherent viscosity results as the quotient of the natural logarithm of the relative viscosity divided by the concentration in grams of the dissolved substance per millimeter solution. There results herefrom the intrinsic viscosity as a limiting value of the inherent viscosity for the case that the aforesaid concentration goes towards zero. The so-called Hagen-Poiseuille's formula is fundamental for viscosity measures. According to the state of the art, individual capillary measures are known for which the volume rate of the solution flow and the flow pressure drop are measured and, the geometric dimensions of the capillary being known, the viscosities of the examined liquids can be determined herefrom. The disadvantage of this measuring method consists in the unfavourable signal-to-noise ratio. The noise is essentially produced by high-frequency interfering signals of the pump which is required for conveying the substance to be examined. Moreover, irregular flow rates of the substance including the counterpressure fluctuations produce interfering signals on flow resistances. Finally, it is known that the viscosity is of course temperature-dependent, for which reason variations of temperatures during the measure can distort the measuring result.
According to the U.S. Pat. No. 3,808,877, to solve this problem, a flow limiter is used between the solvent feeding point and the measuring capillaries to produce a constant flow rate. The relative viscosity is determined by separate measures of the pressure drop on the capillary for the direct flowing polymer solutions and for the pure solvent. From this printed document, a device of two capillaries in parallel running legs is also known, one of them being filled with the polymer solution and the other one with the solvent. Basically, separated measures of said substances are also possible in such a way that the first substance flows through the first capillary and the second substance through the second capillary of a conduit during the measuring, whereby these capillaries are connected in series the one behind the other. The condition for carrying out an exact viscosity determination is in particular the geometric coincidence of the diameter and of the length of the used capillaries, likewise a temperature uniformity at the measuring points.
According to the EP 0 181 224, a capillary viscosimeter is proposed with two capillaries connected in series for which the one serves as a reference capillary only for the solvent and the second as an analysis capillary for the polymer solvent solution. The capillaries consist of long thin tubes into which the solvent is introduced through a pump. A resistance path in form of a tube with a small diameter is between the pump and the reference capillary which serves to produce a counterpressure. A further pulse attenuator can eventually be added to this resistance path. The differential pressure measured in the reference capillary (pressure drop) is supplied to a differential amplifier or to an evaluating unit just as the pressure drop which is measured on the analysis capillary. The feeding point for the substance to be examined, for example a polymer, is between the reference capillary and the analysis capillary so that the analysis capillary is traversed by a solution consisting of the polymer and the solvent. This arrangement connected in series can be changed as far as the feeding point for the test substrate can also be situated before the first capillary.
In this case, the first capillary becomes the analysis capillary. After having passed through it, the solution flows into a retaining vessel which assumes the function of diluting the solution further so that substantially only the solvent is measured by the reference capillary. In the described arrangement, a gel permeation chromatograph can be placed between the feeding point, for example for the polymer, and the analysis capillary, chromatograph in which polymer substances can be separated in a dilution solution according to their molecular size.
Besides the series connection described above, capillary bridge viscosimeters are also still known which are characterized by a relatively high sensitivity. In the bridge connection, a conduit is separated into two parallel running conduit parts in which there are respectively two capillaries connected in series. A place situated between the respectively first and the second capillary of each leg is connected with the corresponding place of the other leg over a connection conduit in which a high sensitive pressure sensor is placed.
According to the embodiment described in the EP 0 113 560, a retention basin in form of a switchable bypass device is moreover provided for before the second capillary of a leg. As far as all existing four capillaries are flown through by the same liquid—and in particular without including the bypass—the connection conduit remains unpressurized. However, if a storage tank is placed above the bypass conduit, the second measuring capillary is substantially only flown through by the solvent so that there results a pressure drop with respect to the other measuring leg because of the different viscosities of the liquids. This pressure drop can be recorded and can be used for determining the viscosity.
Moreover, from the EP 0 083 524, we still know devices with only one capillary which are supposed to have a length of several meters for a diameter between 0.2 and 0.3 mm. This capillary with a total length of, for example 3 mm, is wound in form of a loop with a diameter of at least 10 cm.